The worm and gear speed reducer is one of the most frequently used when a large speed reduction is required. However, it has limitations: low load capacity and not a high efficiency. The low load capacity is inherent to the geometry since the pitch cylinder of the worm has only one point of tangency with the pitch cylinder of the gear and, therefore, with very few teeth in contact. The efficiency is not sufficiently high due to the sliding contact between the worm and the gear with the resulting loss of energy due to friction. The sliding contact also gives rise to rubbing that gives rise to wear. Furthermore, in some applications, the heat generated by friction is such that a cooling system is required and this involves a greater complication and higher manufacturing and maintenance costs.
During the preceding centuries several patents were granted referring to concepts to increase the load capacity of the worm and gear reducers by increasing the contact surfaces. The best known concepts to achieve this goal have been, on the one hand, a gear configuration with the circumferential toothed surface concave shaped so as to partially envelop or wrap the worm, and on the other hand, a worm with a concave silhouette which partially envelops the gear. Also, gear and worm assemblies are used with double enveloping arrangements, that is, which simultaneously incorporate both configurations. All of these schemes have contributed to noticeably raise the load capacity, but are still limited.
In 1897, the concept of a screw and chain speed reducer described in U.S. Pat. No. 595,508 by Wolander appeared, which refers to a speed reducer similar to a worm and gear, such as is shown in FIG. 1, but, instead of the gear, it has a transmission chain with threaded links with a small arc of contact with the screw thread. The turning of the screw drives the threaded links, and thus it also drives the chain which is mounted on two identical sprockets. Since the screw is in contact with said threaded links on the straight portion of the chain, the contact surface may be large, and therefore, the load capacity can be higher than that of the configurations described in the previous paragraph. However, since the motion transmission in all of these arrangements is carried out by means of sliding contact, the efficiency is low. The same can be said of patent DE 2406360, published in August 1975, by Werther, as well as patent JP2000-097293 by Hanaguchi Yuuji, published in April 2000, such as is shown in FIG. 2.
U.S. Pat. No. 418,328, published in December 1889 by Willett refers to a mechanism which drives a boat by means of pedals which includes an angular speed amplifier, made up of a chain equipped with small wheels that drive a “spiral B′”, such as is shown in FIG. 3. In this case, the efficiency is raised by means of the small wheels and there is the potential to somewhat raise the load capacity but it is not really used, and neither is such possibility mentioned. The same is true for U.S. Pat. No. 594,511 published in November 1897, by Auble whose invention refers to a land vehicle.
U.S. Pat. No. 642,430 published in January 1900 by Corcoran, and DE 3305551 C2 published in September 1990 by Reguzzi, such as are shown in FIG. 4, as well as U.S. Pat. No. 7,222,682 published in May 2007 by Doering, such as is shown in FIG. 5, also refer to chains with small wheels, by which efficiency is raised, but there is also an increased load capacity compared to the patents described in the previous paragraph, because unlike them, all the rods in the chain have small wheels.
On the other hand, the patents: U.S. Pat. No. 626,515 published in June 1899, by Whitney and U.S. Pat. No. 747,463 published in December 1903, by Moore, such as are shown in FIG. 6, refer to worm and gear reducers which include small wheels in the gear to reduce friction and increase efficiency but, as has already been mentioned, this type of reducers have few teeth in contact with the consequent limitation on the load capacity.
In the past few decades there has been a general, great interest, in energy savings. Particularly, in the case of worm and gear speed reducers, patents have been granted, and applications continue to be filed which involve concepts to render them more efficient. Therefore, there are patents, for example, U.S. Pat. No. 4,023,433 published on May 9, 1977, by Schutz, such as can be seen in FIG. 7a; and U.S. Pat. No. 7,051,610 published on May 2006, by Stoianovici, such as can be seen in FIG. 7b, which refer to an assembly of balls that roll between the worm thread and the gear teeth, circulating in a closed circuit, that is to say, that once they have journeyed through the entire helicoid thread, they return through an ad hoc conduit to return and re-journey said thread. Of course, the threads are designed in such a way so as to aid in the transit of the balls. In this manner, the sliding contact between the worm and the gear is substituted by a rolling contact which lowers the friction coefficient thereby decreasing the losses and, consequently, increasing the efficiency. In these patents, the gear partially envelops the worm and the worm partially envelops the gear in order to somewhat increase the load capacity, but within the limit imposed by the geometry of the worm and gear assembly, as has been discussed above.
Other patents which could be considered generally relevant in prior art, as far as the roller screw which is applied in the second embodiment of the present invention, are patents U.S. Pat. No. 2,683,379, published in July 1954, by Strandgren, and U.S. Pat. No. 8,082,818, published in December 2011, by Sugitani. However, these patents do not describe a reducer. The conventional roller screw is driven by a tube which has an internal thread, and in the case of the inverted roller screw, said tube is driven by the screw. However, for the sake of simplicity, the inverted roller screw which is used in the second embodiment of the present invention will be simply called “roller screw”. This term refers to an assembly, which basically consists of a worm provided with threaded planetary rollers, and placed within an internally threaded tube. The threaded planetary rollers roll between the worm and the threaded tube, and upon turning they displace said tube on a straight line. In the present invention, the internally threaded tube is substituted by a plurality of nuts which are mounted on the links of a chain transmission.